So last week my buddy finally found a dry day and some time to swap his old battery tray with the one generously donated by RexNICO. As soon as I hovered over the area with the new one in hand, I had a sinking feeling. We pulled out the battery, cover, box, and tray (at this point mostly for instructional purposes) and sure enough the new one was too small.
94cobra (above) had given us hope that we could make it work, but we couldn't find a way to mount the group H7 battery and have everything fit back together correctly. It would work, I'm sure, with a smaller battery.
In removing all the plastics, we did find the specific issue with his original tray- the threads on the retaining bolt aren't stripped, but the metal ring on the underside which holds the threaded cylinder in place as the bolt gets tightened, spins freely, preventing the clamp from being secure against the battery lower lip. I think I might be able to just use some JB Weld or other product to seal the retaining ring against the threaded cylinder, allowing the bolt to tighten again....
Regardless, thanks again for everyone's help, and to RexNICO for the donation. I'll send Fred some money in gratitude. If anyone has better suggestions for fixing the battery tray than my idea, I'm all ears. We are going to do some things to his TDI this weekend, including fixing the common broken plastic in the rear hatch lock actuator (using the part from jetta trunk fix), and a few other minor things if it's not too cold.